1. Field of the Invention
The present invention relates to a printing method and a printer, and more particularly to a printing method and a printer using a discharge medium and a quantitative medium arranged in such a manner that their physical properties have an adequate relationship, whereby exhibiting excellent discharge stability and capable of accurately expressing a gradient image.
More particularly, the present invention relates to a printer of a type for mixing and discharging a quantitative medium and a discharge medium, and more particularly to a printer exhibiting stable discharge and accurate gradation expression because of the specified relationship between the viscosity of a quantitative medium and that of a discharge medium.
2. Description of Related Art
In recent years, an operation called "desktop publishing" for publishing a document by using a computer has widely been performed in business offices and so forth. Moreover, requirements for outputting a natural colored image, such as a photograph, in addition to characters and graphics have been increased. Since New Year's cards, greeting cards and the like have been printed usually in the field of personal use, requirements of the above-mentioned type have been increased. As a result, a high quality and natural image has been required to be printed, thus resulting in gradation expression capable of forming halftone images being made to be important in the above-mentioned circumstance.
A so-called "on-demand" type printer has a structure that ink droplets are discharged from nozzles when required to print an image in accordance with a control signal supplied in response to a recording signal so as to be applied to a recording medium such as a paper sheet or a film so that an image is recorded. Since the on-demand type printer enables the size and cost to furthermore be reduced in the future, the printers of this type have rapidly and widely been employed.
Among a variety of the suggested methods of discharging ink droplets from nozzles, a method using a piezoelectric device or that using a heat generating device has been usually employed. The former method is arranged to use deformation of the piezoelectric device which enables pressure to be applied to ink required to be discharged. The latter method has a structure formed in such a manner that the heating device heats and vaporizes ink in the nozzles to generate bubbles for discharging ink.
To perform the gradient expression realized by forming halftone images in a pseudo manner with the above-mentioned on-demand type printer, a variety of methods have been suggested. A first method has a structure in which the voltage or the width of voltage pulses which are applied to the piezoelectric device or the heating device is changed to control the size of the droplet so as to vary the diameter of a dot to be printed in order to express a gradient image.
However, the first method cannot form a droplet having a satisfactorily small diameter because ink cannot be discharged 3if the voltage or the width of pulses arranged to be supplied to the piezoelectric device or the heating device is reduced excessively. Thus, there arises a problem in that the number of gradients, which can be expressed, is too small. In particular, a low-density image cannot easily be expressed. Therefore, the first method is unsatisfactory to print a natural image.
A second method has a structure that the diameter of the dot is not changed and one pixel is formed by a matrix composed of, for example, 4 dots.times.4 dots so as to express a gradient image in matrix units by performing an image process, such as a dither method or an error diffusion method. In this case, an image processing technique, such as an outline highlighting process and/or a smoothing process is sometimes combined with the second method.
The second method, which is capable of expressing 17 gradients when one pixel is composed of a matrix in the form of 4 dots.times.4 dots, however, encounters deterioration in the resolution to 1/4 if an image is printed with a dot density employed in the first method. In this case, the formed image is too rough to obtain a satisfactory print of a natural image.
If a precise image process is performed to overcome the above-mentioned problem, various problems arise in that circuits become too complicated, the calculation speed is reduced and thus excessively long time is required to complete the printing operation.
Accordingly, inventors of the present invention intended to make clear the principles of the problems experienced with the conventional on-demand type printer have suggested a printer as disclosed in, for example, Japanese Patent Laid-Open No. 5-201024. The printer has a structure in which diluted ink is prepared by mixing ink and diluent, which is a transparent solvent, with each other at a predetermined mixture ratio immediately before discharge so as to immediately discharge diluted ink through nozzles. Thus, a recording medium is applied with the ink droplets so that an image is recorded. Among the above-mentioned methods, the foregoing method, in which ink serving as a quantitative medium and diluent serving as a discharge medium are used such that ink serving as the quantitative medium is mixed with the diluent serving as the discharge medium to prepare diluted ink, and then the discharge medium is discharged to record an image, is called a "carrier jet method". Note that the above-mentioned printer does not arise any problem if the diluent is used as the quantitative medium and ink is used as the discharge medium.
The carrier jet printer is able to control the concentration of the diluted ink droplet, which is discharged, so as to vary the density of each dot which is printed. Therefore, a natural image including a sufficiently large number of halftones can be printed without deterioration in the resolution.
The printer adapted to the carrier jet method exhibits considerably wide degree of freedom in selecting components including ink and the diluent.
As described above, the printer of a type arranged in such a manner that ink and the diluent are mixed with each other so as to be discharged has a requirement for accurately controlling the mixture ratio of ink and the diluent in order to accurately express the gradation corresponding to image data. To achieve this, an operation for allowing the quantitative medium to seep from a second nozzle for discharging the quantitative medium toward a first nozzle for discharging the discharge medium and an operation for discharging the discharge medium from the first nozzle so as to mix and discharge the quantitative medium and the discharge medium must be performed accurately. The stability in discharging the quantitative medium and the discharge medium from each nozzle is a very important factor to obtain a required result of the printing operation.
Although the conventional ink jet printer is simply required to make the viscosity of ink to be suitable for use in the discharging operation because the printer of this type performs only the operation for discharging ink. However, the carrier jet printer, structured to discharge the mixed droplet after it has performed the operation for mixing ink and the diluent with each other, is needed to make the viscosity of each solution to be suitable for use in the mixing operation as well as in the discharging operation.